What Is BMR (Basal Metabolic Rate)?
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep your heart beating, lungs breathing, brain functioning, and cells repairing. Even if you lay in bed all day without moving, your body would still burn this many calories.
BMR accounts for roughly 60–75% of your total daily calorie expenditure. The remaining 25–40% comes from physical activity and the thermic effect of food (energy used to digest what you eat).
Factors That Influence BMR
Your BMR is not a fixed number. It changes based on:
- Muscle Mass — The single biggest factor. Muscle tissue burns 4–7 times more calories at rest than fat tissue. A person with more muscle will have a higher BMR at the same body weight.
- Age — BMR decreases by approximately 1–2% per decade after age 20, largely due to gradual muscle loss (sarcopenia).
- Sex — Men typically have higher BMRs than women of the same height and weight because men carry more lean muscle mass.
- Genetics — Some people are naturally fast metabolizers, others slow. This is largely inherited.
- Hormones — Thyroid hormones are the primary regulators. Hyperthyroidism raises BMR; hypothyroidism lowers it significantly.
- Body Size — Larger bodies have more metabolically active tissue and therefore higher BMRs.
The Three Major BMR Formulas Compared
Our BMR Calculator lets you compare results from all three major scientific formulas:
1. Mifflin-St Jeor Equation (1990) — Recommended
The most accurate formula for the general population, validated in multiple studies:
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
This is considered the gold standard for non-obese individuals and is recommended by the Academy of Nutrition and Dietetics.
2. Harris-Benedict Equation (1919, revised 1984)
The classic formula, still widely used:
- Men: BMR = (13.397 × kg) + (4.799 × cm) − (5.677 × age) + 88.362
- Women: BMR = (9.247 × kg) + (3.098 × cm) − (4.330 × age) + 447.593
It tends to overestimate BMR slightly in modern populations but remains a useful benchmark.
3. Katch-McArdle Formula — Best If You Know Body Fat %
The only formula that accounts for body composition directly:
- BMR = 370 + (21.6 × lean body mass in kg)
If you know your body fat percentage (from a DEXA scan, calipers, or bioelectrical impedance scale), this formula is the most accurate because it calculates from your actual muscle mass rather than total weight. It is especially useful for athletes and bodybuilders.
From BMR to TDEE: Adding Activity
BMR is your resting burn. TDEE (Total Daily Energy Expenditure) is your total daily burn including all movement. To calculate TDEE, multiply BMR by an activity factor:
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Little or no exercise, desk job |
| Lightly Active | 1.375 | Light exercise 1–3 days/week |
| Moderately Active | 1.55 | Moderate exercise 3–5 days/week |
| Very Active | 1.725 | Hard exercise 6–7 days/week |
| Extra Active | 1.9 | Very hard exercise, physical job, or 2x/day training |
Example: If your BMR is 1,600 calories and you are moderately active (multiplier 1.55), your TDEE is 1,600 × 1.55 = 2,480 calories per day. This is the number you need to eat to maintain your current weight.
Using BMR and TDEE for Your Goals
Once you know your TDEE, you can set a daily calorie target:
| Goal | Calorie Target | Expected Result |
|---|---|---|
| Fast Weight Loss | TDEE − 1,000 | ~1 kg (2 lbs) per week |
| Moderate Weight Loss | TDEE − 500 | ~0.5 kg (1 lb) per week |
| Maintain Weight | TDEE (exact) | Stable weight |
| Muscle Gain (Lean Bulk) | TDEE + 250–500 | ~0.25–0.5 kg muscle per week |
| Fast Weight Gain | TDEE + 1,000 | ~1 kg (2 lbs) per week |
Important: Never eat below your BMR for extended periods — this causes muscle loss, metabolic slowdown, and nutritional deficiencies. The minimum safe intake is 1,200 calories/day for women and 1,500 for men.
Does BMR Change Over Time?
Yes, and significantly. Here is what happens and what you can do about it:
- After 30: Sedentary adults lose 3–5% of muscle mass per decade. This lowers BMR.
- Menopause: Women experience a significant BMR drop due to hormonal changes.
- Crash Dieting: Extreme calorie restriction can lower BMR by 15–30% through a survival mechanism called adaptive thermogenesis. This is why dieters often regain weight — their metabolism has slowed.
The solution: Regular strength training preserves and builds muscle, which keeps your BMR elevated. Even 2–3 sessions per week can offset age-related metabolic decline.
Try All Three Formulas Free
Use the BMR Calculator to compare Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle results side by side. Get your personalized calorie target for weight loss, maintenance, or muscle gain — all calculations run in your browser with zero data collection.